IANAL & I live in the UK.
However, if the shipping company and the power plant loose money as a result of the action, why not claim for the losses? Nobody is astopping freedom of speech and freedom of demonstration but if I were to cause Greenpeace to loose money by deliberately obstructing their lawful activities I bet they would have me in court. And fast.

As long as they aren’t being violent, they aren’t valid targets. However, the first time that someone picks up a rock and throws it or they ram the ship, they become pirates and can legally be slain by anyone who meets them.